For example, radar devices generate electromagnetic waves in a microwave source such as a magnetron, guide the electromagnetic waves to an antenna via one or more waveguides, and then emit the electromagnetic waves to the outside from the antenna. In a case where the electromagnetic waves are propagated (transmitted) from a rectangular cross section upstream waveguide to a rectangular cross section downstream waveguide where cross-sectional orientations and extending directions of the upstream and downstream waveguides differ therebetween, an electromagnetic wave coupling (converting) structure is to be adopted, in order to install the waveguides in a narrow space. For example, the following configuration is known. If the electromagnetic waves are propagated from a vertically arranged waveguide to a transversely arranged waveguide, a loop probe having a given diameter is arranged within the vertical waveguide, at a suitable location on a tube wall of the waveguide. Further, an electric field probe set to another given diameter is arranged in the transverse waveguide side. Thus, an impedance matching is obtained between both the waveguides to enable a coupling of the electromagnetic waves.
WO2007/035523 discloses that a radar device for emitting electromagnetic waves to the outside through a waveguide adopts the above-mentioned coupling technique. In FIGS. 6 and 7 of WO2007/035523, a configuration in which a signal coupler is provided between waveguide sections is illustrated. The signal coupler includes a coaxial connector, and also includes a conductive-wire loop probe made of a highly conductive material which extracts or feeds electromagnetic waves from/to the waveguide. The coaxial connector includes a central conductive wire and a cylindrical insulated spacer both having a predetermined length, and an impedance matching of the transmission path is obtained by suitably designing sizes and the like of these components.
However, it is difficult to design a diameter of the transverse cross-section of the probe mentioned above in order to obtain the impedance matching. Moreover, because of the size of the diameter of the loop probe portion, the loop shape could not be easily achieved and, thus, it requires processing such as partially cutting the loop portion. Therefore, the manufacture is not easy while the structure is complicated. In addition, the signal coupler disclosed in WO2007/035523 is not structurally simple, either.